As an experiment since I'm a sucky new silver player my son who is diamond did my placements and surprise surprise it makes no difference still placed silver even though he completely wrecked for the 10 placement matches.
Have you played previous seasons? They tend to group you into your previous skill bracket. Even last season I lost 9 placement matches and still got put at 3400 which is silly. Game's a grind.
they really need to put something in place like DOTA2/League have, if you leave or get reported you get put in a low priority no ranked Queue for a number of games or time. Leavers are the plague of overwatch